Method and apparatus for transmitting control information in WLAN system

ABSTRACT

There is provided a method of transmitting control information in a Wireless Local Area Network (WLAN) system, comprising transmitting first control information by means of cyclic shift delay diversity beam-forming and transmitting second control information. The first control information comprises information necessary for each of a plurality of target stations of the second control information to receive the second control information. The second control information beamformed and transmitted to the plurality of target stations.

This application is the National Phase of PCT/KR2010/006093 filed onSep. 8, 2010, which claims priority under 35 U.S.C. 119(e) to U.S.Provisional Application No.(s) 61/240,658 filed on Sep. 9, 2009,61/243,160 filed on Sep. 17, 2009, 61/259,634 filed on Nov. 9, 2009,61/303,684 filed on Feb. 12, 2010, 61/307,429 filed on Feb. 23, 2010 and61/349,220 filed on May 28, 2010, and under U.S.C. 119(a) to PatentApplication No.(s) 10-2010-0022225 filed in the Republic of Korea onMar. 12, 2010, 10-2010-0040589 filed on Apr. 30, 2010 and10-2010-0040588 filed on Apr. 30, 2010, all of which are herebyexpressly incorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus for transmitting controlinformation in a Wireless Local Area Network (WLAN) system supportingmultiple antennas.

BACKGROUND ART

With the recent development of information communication technology, avariety of wireless communication techniques are being developed. Fromamong them, a WLAN is a technique which wirelessly enables access to theInternet at home or companies or in a specific service providing areausing mobile terminals, such as a Personal Digital Assistant (PDA), alaptop computer, and a Portable Multimedia Player (PMP), on the basis ofradio frequency technology.

Since Institute of Electrical and Electronics Engineers (IEEE) 802(i.e., the standard organization of WLAN technology) has been set upFebruary, 1980, lots of standardization task are being performed.

The initial WLAN technology was able to support the rate of 1 to 2 Mbpsthrough frequency hopping, band spreading, and infrared communicationusing a 2.4 GHz frequency band in accordance with IEEE 802.11, butrecently can support the maximum rate of 54 Mbps using OrthogonalFrequency Division Multiplex (OFDM). In addition, in IEEE 802.11, thestandardization of various techniques, such as the improvement ofQuality for Service (QoS), Access Point (AP) protocol compatibility,security enhancement, radio resource measurement, wireless accessvehicular environment for vehicle environments, fast roaming, a meshnetwork, interworking with an external network, and wireless networkmanagement, is being put to practical use being developed.

IEEE 802.11b from the IEEE 802.11 supports a maximum transmission speedof 11 Mbs while using the 2.4 GHz frequency band. IEEE 802.11acommercialized since the IEEE 802.11b has reduced the influence ofinterference as compared with the very complicated 2.4 GHz frequencyband by using a 5 GHz frequency band not the 2.4 GHz frequency band andalso improved the transmission speed up to a maximum of 54 Mbps usingthe OFDM technique. However, the IEEE 802.11a is disadvantageous in thatthe transmission distance is shorter than that of the IEEE 802.11b.Further, IEEE 802.11 g implements a maximum transmission speed of 54Mbps using the 2.4 GHz frequency band like the IEEE 802.11b, and it issignificantly being in the spotlight because it satisfies backwardcompatibility. The IEEE 802.11 g is superior to the IEEE 802.11a even inthe transmission distance.

Further, as a technique for overcoming the limit to the transmissionspeed pointed out as vulnerabilities in the WLAN, there is IEEE 802.11nwhich has recently been standardized. The IEEE 802.11n has its object toincrease the speed and reliability of a network and to expand theoperating distance of a wireless network. More particularly, the IEEE802.11n is configured to support a High Throughput (HT) having a dataprocessing speed of a maximum of 540 Mbps or more and based on aMultiple Inputs and Multiple Outputs (MIMO) technique using multipleantennas on both sides of a transmitter and a receiver in order tominimize transmission error and optimize the data rate. Further, theIEEE 802.11n may use a coding method of transmitting several redundantcopies in order to increase the reliability of data and OFDM (OrthogonalFrequency Division Multiplex) in order to increase the speed.

With the wide spread of the WLAN and various applications using theWLAN, a necessity for a new WLAN system for supporting a higherthroughput than the data processing speed supported by the IEEE 802.11nis recently gathering strength. A Very High Throughput (VHT) WLAN systemis one of IEEE 802.11 WLAN systems which have recently been newlyproposed in order to support the data processing speed of 1 Gbps ormore. The name of the VHT WLAN system is arbitrary, and a feasibilitytest for a system using 4×4 MIMO and a channel bandwidth of 80 MHz ormore in order to provide the throughput of 1 Gbps or more is beingperformed.

The VHT WLAN system now being discussed includes two kinds of methodsusing a frequency band of 6 GHz or less and a frequency band of 60 GHz.If the frequency band of 6 GHz or less is used, a possibility ofcoexistence with conventional WLAN systems using the frequency band of 6GHz or less can become problematic.

Meanwhile, the physical (PHY) layer architecture of the IEEE 802.11consists of a PHY Layer Management Entity (PLME), a Physical LayerConvergence Procedure (PLCP) sublayer, and a Physical Medium Dependent(PMD) sublayer. The PLME functions to manage the physical layer whilecooperating with a MAC Layer Management Entity (MLME). The PLCP sublayerfunctions to transfer a MAC Protocol Data Unit (MPDU), received from theMAC layer, to the PMD sublayer or transfers frames, received from thePMD sublayer, to the MAC layer between the MAC layer and the PMD layerin accordance with an instruction of the MAC layer. The PMD sublayer isa lower layer of the PLCP and it enables the transmission and receptionof a physical layer entity between two stations through a radio medium.

The PLCP sublayer attaches additional fields, including informationnecessary for a physical layer transceiver, to an MPDU in a process ofreceiving the MPDU from the MAC layer and sending the MPDU to the PMDsublayer. The fields attached in this case can include a PLCP preamblefor the MPDU, a PLCP header, tail bits over a data field, and so on. ThePLCP preamble functions to have a receiver prepare for a synchronizationfunction and antenna diversity before a PSDU (PLCP Service DataUnit=MPDU) is transmitted. The PLCP header includes information about aframe (e.g., PSDU Length Word (PLW)), information about the data rate ofa PSDU portion, and information about header error check.

The PLCP sublayer generates a PLCP Protocol Data Unit (PPDU) by addingthe above fields to the MPDU and sends the PPDU to a reception stationvia a PMD sublayer. The reception station restores data by acquiring thePLCP preamble of the received PPDU and information about datarestoration from the PLCP header.

In case where a variety of legacy stations and VHT stations, such asIEEE 802.11a/b/g/n, coexist, the legacy station cannot recognize orerroneously recognize the PLCP format and thus can malfunction. In orderto prevent the above problem, in case where the PLCP format recognizableby the legacy stations and a format for the VHT stations are attached toall transmission data so that the formats can be recognized by all thestations, overhead is increased, thus hindering the efficient use ofradio resources. Further, in a WLAN system supporting Multi-User(MU)-MIMO, in case where radio frames are spatially multiplexed formultiple users and transmitted, there is a problem that a station (i.e.,not a target of transmission) cannot recognize the radio frames. It isalso expected that the amount of control information necessary to send,receive, and decode data will be increased according to the MU-MIMOsupport.

Consideration is required for a new frame format for a method oftransmitting control information in a WLAN system supporting MU-MIMO andfor a VHT WLAN system which can accommodate increasing controlinformation, support backward compatibility, and guarantee coexistencewith a legacy station.

SUMMARY OF INVENTION Technical Problem

It is an object of the present invention to provide a method oftransmitting control information in a WLAN system supporting MU-MIMO.

It is another object of the present invention to provide a method ofaccommodating control information and transmitting frames in a WLANsystem supporting MU-MIMO.

Solution to Problem

In an aspect, a method of transmitting control information in a WirelessLocal Area Network (WLAN) system includes transmitting first controlinformation by means of cyclic shift delay diversity beam-forming, andtransmitting second control information, wherein the first controlinformation comprises information necessary for each of a plurality oftarget stations of the second control information to receive the secondcontrol information, and the second control information is beamformedand transmitted to the plurality of target stations.

The first control information may further include information about atransmission time taken to transmit spatially multiplexed SpatialDivision Multiple Access (SDMA) data to the plurality of targetstations.

The second control information may include control information abouteach of the plurality of target stations.

The control information about each of the plurality of target stationsmay include at least one of Modulation and Coding Scheme (MCS)information, channel bandwidth information, information about a numberof spatial streams, and transmission power information.

The first control information and the second control information may betransmitted through a first frame, and the second control informationmay include information about a transmission time taken to transmit oneor more second frames subsequent to the first frame.

A number of subcarriers per Orthogonal Frequency Division Multiplexing(OFDM) symbol allocated to transmit the first control information may besmaller than a number of subcarriers per OFDM symbol allocated totransmit the second control information.

A number of OFDM symbols allocated to transmit public controlinformation may be greater than a number of OFDM symbols allocated totransmit STA-specific control information.

The first control information and the second control information areapplied to different cyclic shifts.

Advantageous Effects of Invention

There are provided a control information transmission method and a PLCPframe format which can be applied to a WLAN system supporting MU-MIMO.Further, the coexistence of a VHT station and a legacy station isguaranteed because backward compatibility is supported.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of a WLAN system to which anembodiment of the present invention can be applied;

FIG. 2 shows examples of the existing PLCP frame format;

FIG. 3 is a block diagram showing an example of a PLCP frame formataccording to an embodiment of the present invention;

FIG. 4 shows an example of a PLCP frame applied to a VHT systemaccording to the present invention;

FIG. 5 shows the allocation of resources used to transmit VHTSIG-A andVHTSIG-B;

FIG. 6 shows an example of a BPSK constellation for VHTSIG-A;

FIG. 7 is a flowchart illustrating a method of transmitting controlinformation according to an embodiment of the present invention;

FIGS. 8 to 37 are block diagrams showing examples of a PLCP framestructure and a transmission method according to some embodiments of thepresent invention; and

FIG. 38 is a block diagram showing an embodiment of a radio apparatus inwhich an embodiment of the present invention is implemented.

MODE FOR THE INVENTION

Some embodiments of the present invention are described in detail withreference to the accompanying drawings.

FIG. 1 is a diagram showing an example of a WLAN system to which anembodiment of the present invention can be applied.

Referring to FIG. 1, the WLAN system includes one or more Basic ServiceSets (BSSs). The BSS is a set of stations (STAs) which are successfullysynchronized with other each and can perform transmission reciprocally.The BSS is not a concept indicative of a specific area. Further, a BSSsupporting ultra-high data processing of 1 GHz or more in an MAC SAP,such as the WLAN system to which an embodiment of the present inventioncan be applied, is referred to as a Very High Throughput (VHT) BSS.

The VHT BSS can be classified into an infrastructure BSS and anindependent BSS (IBSS). Infrastructure BSSs are shown in FIG. 1. Theinfrastructure BSSs BSS1 and BSS2 include one or more Non-AP STAs STA 1,STA 3, and STA 4, APs AP 1 (STA 2) and AP 2 (STA 5) (i.e., stationsproviding distribution service, and a Distribution System (DS)interconnecting the plurality of APs AP 1 and AP 2. In theinfrastructure BSS, the AP STA manages the Non-AP STAs of the BSS.

On the other hand, the independent BSS (IBSS) is operated in the ad-hocmode. The IBSS does not include a centralized management entity becauseit does not include an AP VHT STA. That is, in the IBSS, Non-AP STAs aremanaged in a distributed manner. In the IBSS, all STAs can be composedof mobile STAs, and the STAs form a self-contained network because theyare not permitted to access a DS.

An STA is a certain function medium, including Medium Access Control(MAC) according to the IEEE 802.11 standards and a physical layerinterface for a radio medium. The STA includes both an AP and a Non-APSTA in a broad sense. Further, in a multi-channel environment to bedescribed later, an STA supporting ultra-high data processing of 1 GHzor more is also referred to as a VHT STA. In a VHT WLAN system to whichan embodiment of the present invention can be applied, all STAs includedin the above BSS may be VHT STAs, or VHT STAs and legacy STAs (e.g., HTSTAs according to IEEE 802.11a/b/g/n) can coexist in the above BSS.

An STA for wireless communication includes a processor and a transceiverand can further include a user interface, display means, and so on. Theprocessor is a function unit designed to generate frames to betransmitted over a wireless network or to process frames received overthe wireless network. The processor performs several functions forcontrolling STAs. Further, the transceiver is functionally coupled tothe processor and is a unit designed to transmit and receive frames overa wireless network for STAs.

Portable terminals manipulated by users, from among STAs, are Non-APSTAs STA1, STA3, STA4, and STA5. Assuming that the mobile terminals aresimply STAs, they also refer to Non-AP STAs. The Non-AP STA may bereferred to as another terminology, such as a terminal, a WirelessTransmit/Receive Unit (WTRU), a User Equipment (UE), a Mobile Station(MS), a mobile terminal, or a Mobile Subscriber Unit. Further, a Non-APSTA supporting ultra-high data processing on the basis of the MU-MIMOtechnique to be described later is referred to as a Non-AP VHT STA (orsimply a VHT STA).

Further, the APs AP1 and AP2 are function entities which provide accessto the DS via a radio medium for STAs associated therewith. It is aprinciple that in an infrastructure BSS including an AP, communicationbetween Non-AP STAs is performed via the AP. However, in case where adirect link is established, such communication can be directly performedbetween the Non-AP STAs. The AP can be referred to as anotherterminology, such as a centralized controller, a Base Station (BS), anode-B, a Base Transceiver System (BTS), or a side controller, inaddition to an access point. Further, an AP supporting ultra-high dataprocessing on the basis of the MU-MIMO technique to be described lateris called a VHT AP.

A plurality of infrastructure BSSs can be interconnected through aDistribution System (DS). The plurality of BSSs interconnected throughthe DS is called an Extended Service Set (ESS). STAs included in the ESScan communicate with each other. Non-AP STAs within the same ESS canmove from one BSS to another BSS while seamlessly communicating witheach other.

The DS is a mechanism for allowing one AP to communicate with anotherAP. In case where an AP sends frames for STAs associated with a BSSmanaged by the AP or any one STA moves to another BSS, the DS cantransfer the frames or transfer the frames over an external network,such as a wired network. The DS needs not to be necessarily a networkand can include any type as long as it can provide a certaindistribution service defined in the IEEE 802.11. For example, the DS maybe a wireless network, such as a mesh network, or a physical structureinterconnecting APs.

Meanwhile, a VHT WLAN system uses MU-MIMO so that several STAs canefficiently use wireless channels simultaneously. In other words, theVHT WLAN system allows several STAs to perform transmission andreception to and from an AP at the same time. The AP can send aspatially multiplexed radio frame to several STAs at the same time. Tothis end, the AP may perform beam-forming by measuring channelsituations and may transmit and receive data using a plurality ofspatial streams.

Hereinafter, to transmit the multiplexed data to a plurality of STAs isreferred to as MU-MIMO transmission or SDMA transmission. In MU-MIMOtransmission, at least one spatial stream is allocated to each of theSTAs (i.e., targets of transmission), and data can be transmitted usingthe allocated spatial stream.

Hereinafter, a conventional STA (i.e., Non-VHT STA) is referred to as alegacy STA. The legacy STA includes a Non-HT STA supporting IEEE802.11a/b/g standards and an HT STA supporting IEEE 802.11n standards.In various PLCP frame formats proposed by the present invention anddescribed later, fields denoted by the same name, unless speciallymentioned, have the same function in the entire specification.

A PLCP frame according to the PLCP frame format proposed by the presentinvention is generated in the PLCP sublayer of an STA and sent to atransmission target STA using a PLCP frame transmission method, proposedby the present invention, through multiple antennas via a PMD sublayer.Hereinafter, the PLCP frame format and a method of transmitting fieldsconstructing the same, which are described with reference to theaccompanying drawings, are examples of various embodiments of thepresent invention, and the transmission sequence of the fields is notlimited to that shown in the drawings. In the following description, thetransmission sequence, unless not specially described, can be changed,and some fields can be omitted or added by necessity. The PLCP frameformat and the method of transmitting the same to be described later canbe adaptively selected and used according to the types and number ofSTAs constituting a BSS, the amount of data to be transmitted, priority,and so on.

FIG. 2 shows examples of the existing PLCP frame formats. For the PLCPframe formats, reference can be made to sub-clause 20.3 of “Part 11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)specifications; Amendment 5: Enhancements for Higher Throughput” of IEEE802.11n/D11.0 disclosed June, 2009.

The IEEE 802.11n standards provide three types of Physical LayerConvergence Procedure (PLCP) frames, including a Non-HT format, anHT-mixed format, and an HT-greenfield format. The PLCP frame is used totransmit a PLCP Protocol Data Unit (PPDU).

Elements included in the PLCP frame are listed in the following table.

TABLE 1 Element Description L-STF (Non-HT Short Training Used for frametiming acquisition and Field) automatic gain control (AGC) convergenceL-LTF (Non-HT Long Training Used for channel estimation Field) L-SIG(Non-HT SIGNAL Including information for demodulating field) anddecoding data for L-STA HT-SIG (HT SIGNAL field) Including informationfor HT-STA to demodulate and decode data HT-STF (HT Short Training Usedfor frame timing acquisition and Field) AGC convergence HT-GF-STF(HT-Green Field Used for frame timing acquisition and Short TrainingField) AGC convergence (readable by only HT STA) HT-LTF1 (First HT LongUsed for channel estimation Training Field) HT-LTF (HT Long TrainingIncluding data HT-LTF used for Field) channel estimation for datademodulation for HT-STA and extension HT-LTF used for channel soundingData field Including PHY Service Data Unit (PSDU)

The Non-HT format is used for an L-STA, and it includes an L-STF, anL-LTF, and an L-SIG.

The HT-mixed format is used when an HT-STA and an L-STA coexist. Inorder to provide an L-STA with backward compatibility, the L-STF, theL-LTF, and the L-SIG are first sequentially. The HT-SIG is used for anHT-STA to decode data.

The HT-greenfield format is used in a system composed of only HT-STAs.That is, a L-STA cannot receive a PLCP frame that follows theHT-greenfield format.

Short Training Fields (STFs), such as the L-STF, the HT-STF, and theHT-GF-STF, are used for frame timing acquisition, AGC (automatic gaincontrol), etc. and thus are also referred to a synchronization signal ora synchronization channel. That is, the STF is used to meetsynchronization between STAs or an STA and an AP.

Long Training Fields (LTFs), such as the L-LTF and the HT-LTF, are usedfor channel estimation for the demodulation of data or controlinformation or both and thus are also referred to a reference signal, atraining signal, or a preamble.

The L-SIG and the HT-SIG are referred to as control information becausethey provide several pieces of information necessary to decode data.

FIG. 3 is a block diagram showing an example of the PLCP frame formataccording to an embodiment of the present invention.

A VHT PLCP frame 300 includes a VHTSIG-A field 330, a VHTSIG-B field340, and a DATA field 360. Each of the VHTSIG-A field 330 and theVHTSIG-B field 340 includes control information which is necessary for areception STA to demodulate and decode the DATA field 360. The names ofthe VHTSIG-A field 330 and the VHTSIG-B field 340 are arbitrary and canbe represented in various ways by first control information and secondcontrol information, respectively, or a first control signal and asecond control signal, respectively.

The VHTSIG-A field 330 further includes common information about MU-MIMOtransmission of fields to be subsequently transmitted. The VHTSIG-Afield 330 can be transmitted so that all STAs within a BSS can receivethe VHTSIG-A field 330. The VHTSIG-A field 330 may include informationabout a target STA of the VHTSIG-B field 340 to be subsequentlytransmitted and information necessary to receive the VHTSIG-B field 340.The VHTSIG-A field 330 can further include common information in thetransmission of data to the target STA. For example, the VHTSIG-A field330 may include information indicating an SDMA transmission time,together with information about a channel bandwidth used, modulation andcoding information, and information about the number of spatial streamsused. The SDMA transmission time is the time that Spatial DivisionMultiple Access (SDMA) data (i.e., a spatially multiplexed data framefor a plurality of STAs) is taken to be transmitted, and it can bereferred to as an MU-MIMO transmission time. An STA other than a targetof transmission can receive information indicating the SDMA transmissiontime, set a Network Allocation Vector (NAV) for the correspondingtransmission time, and defer channel access.

The VHT-SIG B field 340 includes a parameter value which is used forSDMA transmission every target STA. For example, the VHT-SIG B field 340may include information about parameter values which may be differentlyset according to an individual STA, such as an MCS index valueindicating a Modulation and Coding Scheme (MCS) used, the bandwidth of achannel, and a value indicating the number of spatial streams.

The DATA field 360 includes SDMA-precoded data which will be transmittedto an STA (i.e., a target of transmission) and may further include tailbits or a bit padding element or both by necessity.

The VHT PLCP frame 300 may further include one or more fields, includinginformation for performing frame timing acquisition and AGC convergenceand for selecting diversity and information for channel estimation. Theone or more fields may have a format recognizable by a legacy STA and anHT STA or may have the field of a format, recognizable by a legacy STAand an HT STA, added thereto.

A transmission station which transmits the VHT PLCP frame 300 transmitsthe VHTSIG-A field 330 omni-directionally without SDMA precoding, andapplies SDMA precoding and beam-forming to the VHTSIG-B field 340 andthe subsequent DATA field 360 and transmits them. In the presentinvention, transmission of signals omni-directionally may betransmission of signals using time domain cyclic delay diversitybeam-forming, where signals transmitted in each transmit antenna aretime domain cyclic shifted signals within an OFDM symbol of othertransmit antennas.

The STAs of a BSS receive the VHTSIG-A field 330, transmittedomni-directionally, without SDMA precoding. An STA not belonging totargets of transmission can set an NAV during a period indicated by theSDMA transmission time information included in the VHTSIG-A field 330and defer channel access. An STA belonging to the targets oftransmission can acquire information individualized therefore from theVHTSIG-B field 340 and can receive, demodulate, and decode datatransmitted thereto.

FIG. 4 shows an example of a PLCP frame applied to a VHT systemaccording to the present invention.

The PLCP frame includes an L-STF 410, an L-STF 420, an L-SIG 420,VHTSIG-A 440, a VHT-STF 450, VHT-LTFs 460, VHTSIG-Bs 470, and data 480.

The L-STF 410 is used for frame timing acquisition, AGC (automatic gaincontrol) control, coarse frequency acquisition, etc.

The L-LTF 420 is used for channel estimation for demodulating the L-SIG420 and the VHTSIG-A 440.

The VHT-STF 450 is used for a VHT-STA in order to improve AGC estimationin an MIMO system.

A plurality of the VHT-LTFs 460 is included and used for channelestimation for demodulating the VHTSIG-B 470 and the data 480. TheVHT-LTF 460 can also be referred to as a data VHT-LTF. In addition, anextension VHT-LTF for channel sounding can be used.

Beamforming is not applied to the L-STF 410, the L-LTF 420, the L-SIG430, and the VHTSIG-A 440. Meanwhile, beam-forming for MU-MIMO isapplied to the VHT-STF 450, the VHT-LTFs 460, the VHTSIG-Bs 470, and thedata 480. In the beam-forming, each field is processed through the sameprecoding matrix (or precoding vector). Since the data 480 and theVHT-LTFs 460 are processed through the same precoding matrix, a VHT-STAcan directly demodulate or decode the data 480 through a channelestimated using the VHT-LTF 460 although it does not know the precodingmatrix.

Different cyclic shifts can be applied to a region not subjected tobeam-forming and a region subjected to beam-forming, within a PLCPframe. That is, a first cyclic shift can be applied to the L-STF 410,the L-LTF 420, the L-SIG 430, and the VHTSIG-A 440, and a second cyclicshift can be applied to the VHT-STF 450, the VHT-LTFs 460, and theVHTSIG-B 470.

The cyclic shift can be applied to each OFDM symbol. Further, the cyclicshift can be given every transmission chain.

For example, assuming that a cyclic shift amount T_(cs) is applied to asignal s(t) of an interval 0≦t≦T, a cyclically shifted signal s_(cs)(t)can be defined as follows.

$\begin{matrix}{{s_{cs}\left( {t;T_{cs}} \right)} = \left\{ \begin{matrix}{s\left( {t - T_{cs}} \right)} & {0 \leq t < {T + T_{cs}}} \\{s\left( {t - T_{cs} - T} \right)} & {{T + T_{cs}} \leq t \leq T}\end{matrix} \right.} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

Each PSDU included in the data 480 through beam-forming is transmittedto each STA.

For a VHT-STA, two kinds of control information, including the VHTSIG-A440 and the VHTSIG-Bs 470, are included in the PLCP frame. The VHTSIG-A440 indicates public control information (or also called first controlinformation) for allowing the VHTSIG-B 470 to be received by each STA.The VHTSIG-B 470 indicates STA-specific control information (or calledsecond control information) for allowing each STA to demodulate ordecode or both its own data 480.

The public control information can include at least one of the followingfields.

TABLE 2 Field Name Description SIG-B Length Indicate the length ofVHTSIG-B MU-MIMO Indicator Indicate whether MU-MIMO is used or cantoggle SU-MIMO/MU-MIMO Bandwidth Indicate the bandwidth of a channel STAIndicator Indicate an STA which will receive VHTSIG-B. It can indicatethe address of an STA or indicate the ID of an STA or the index ofVHTSIG-B Number of multiplexings The number of STAs (users) multiplexedthrough MU-MIMO Decoding indicator Indicate information for decodingVHTSIG-B

The STA-specific control information (or also called user-specificcontrol information) can include at least one of the following fields.

TABLE 3 Field Name Description MCS Indicate MCS (modulation and codingscheme) information necessary to decode data STA ID Indicate an STAwhich will use MCS

In Tables 2 and 3, the field names are only illustrative and anothername can be used. The fields of Tables 2 and 3 are only illustrative,some of the fields can be omitted, and other fields can be further addedthe fields.

FIG. 5 shows the allocation of resources used to transmit VHTSIG-A andVHTSIG-B.

Assuming that a bandwidth of 20 MHz is used, an L-STF, an L-LTF, anL-SIG, and VHTSIG-A in which beam-forming is not used uses 52subcarriers (called narrowband (NB) subcarriers) every OFDM (OrthogonalFrequency Division Multiplexing) symbol in order to support an L-STA.The 52 NB subcarriers can be classified into 48 data NB subcarriers and4 pilot NB subcarriers.

A VHT-LTF and VHTSIG-B in which beam-forming is used uses samesubcarriers of DATA field OFDM symbols, which are 56 subcarriers (calledwideband (WB) subcarriers) every OFDM symbol. The 56 WB subcarriers canbe classified into 52 data WB subcarriers and 4 pilot WB subcarriers.

An L-STA uses 52 subcarriers every OFDM symbol in a 20 MHz band. Inorder to provide backward compatibility, the VHTSIG-A uses the samenumber of subcarriers as the L-STF and the L-LTF.

The L-STF uses Quadrature Phase Shift Keying (QPSK) modulation, and itcan be represented by the sequence S of the following frequency domainin one OFDM symbol.

S_(−26,26)=K{0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,0,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0}

where K is a QPSK normalization factor and K=√{square root over (½)}

.A DC subcarrier is not used.

The L-LTF can be represented by the sequence T of the followingfrequency domain in one OFDM symbol.

T_(−26,26)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}

The L-SIG uses Binary Phase Shift Keying (BPSK) modulation and can have48 bits because 48 data NB subcarriers are allocated thereto. When theL-SIG has a 1/2 code rate, the number of information bits of the L-SIGis 24.

The VHTSIG-A also uses BPSK modulation and can have 48 bits every OFDMsymbol because 48 data NB subcarriers are allocated thereto. When theVHTSIG-A has a 1/2 code rate and 2 OFDM symbols allocated thereto, thenumber of information bits of the VHTSIG-A is 48.

In order to facilitate the detection of the VHTSIG-A, a BPSKconstellation for the VHTSIG-A can be rotated around a BPSKconstellation for the L-SIG.

FIG. 6 shows an example of BPSK constellation for VHTSIG-A.

In FIG. 6, the BPSK constellation for VHTSIG-A has been rotated by 90degrees around a BPSK constellation for an L-SIG. This is called arotated constellation. However, this is only illustrative, and therotation angle can be 45 degrees, 180 degrees, and the like. Further,such rotation can be applied to not only BPSK, but also QPSK, 8-PSK, and16-QAM.

Referring back to FIG. 4, the VHT-STF, the VHT-LTF, and the VHTSIG-B inwhich beam-forming is used do not need to maintain compatibility with anL-STA, and use 56 subcarriers every OFDM symbol in order to increasefrequency efficiency.

The VHT-STF uses QPSK (Quadrature Phase Shift Keying) modulation and canbe defined as the following sequence VHTS in one OFDM symbol.

VHTS_(−28,28)=K{0,0,0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,0,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0}

where K is a QPSK normalization factor and K=√{square root over (½)}.

The VHT-LTF can be represented by the sequence VHTT of the followingfrequency domain in one OFDM symbol.

VHTT_(−28,28)={1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,−1,−1}

The VHTSIG-B is mapped using the same modulation as the VHTSIG-A (i.e.,a rotated BPSK constellation), and it uses one OFDM symbol. The VHTSIG-Buses 52 data subcarriers every OFDM symbol. Accordingly, when theVHTSIG-B has a 1/2 code rate, the number of information bits of theVHTSIG-B is 26.

In order to support MU-MIMO, control information necessary to decodedata is divided into VHTSIG-A and VHTSIG-B. The VHTSIG-A is transmittedin omni-directionally and the VHTSIG-B is transmitted directionally. Inother words, for transmission of the VHTSIG-A, channel specificbeam-forming is not used (but cyclic delay shifted beam-forming may beused) and for transmission of the VHTSIG-B, beam-forming is used.

The number of subcarriers (e.g., 52) allocated to the VHTSIG-A issmaller than the number of subcarriers (e.g., 56) allocated to theVHTSIG-B. This means that frequency domain resources allocated to theVHTSIG-A are smaller than frequency domain resources allocated to theVHTSIG-B. This is because the VHTSIG-A is decoded using the channelestimation of an L-LTF in order to maintain backward compatibility.

The number of OFDM symbols (e.g., 2) allocated to the VHTSIG-A isgreater than the number of OFDM symbols (e.g., 1) allocated to theVHTSIG-B. This means that time domain resources allocated to theVHTSIG-A is greater than time domain resources allocated to theVHTSIG-B. This is because if more STAs are multiplexed using MU-MIMO,only one OFDM symbol can be insufficient to transmit the VHTSIG-A.

When an L-STA and a VHT-STA coexist in a 20 MHz bandwidth, differenttime resources or different frequency resources or both can be allocatedto a region providing backward compatibility and a region not providingbackward compatibility. The time and frequency domains supported by allNon-AP STAs and APs within a BSS are allocated to an STF, an LTF, andpublic control information which are transmitted in the region providingbackward compatibility. The time and frequency domains supported by onlya VHT-STA or a VHT-AP are allocated to an STF, an LTF, and STA-specificcontrol information in the region not providing backward compatibility.Accordingly, backward compatibility can be guaranteed, and higherfrequency efficiency can be provided to a VHT-STA supporting MU-MIMO.

In the above structure, a time domain waveform for 20 MHz VHTSIG-A canbe represented by the following equation.

$\begin{matrix}{{r_{{VHTSIG} - A}^{i_{rx}}(t)} = {\frac{1}{\sqrt{N_{TX} \cdot N_{{VHTSIG} - A}^{Tone}}}{\sum\limits_{n = 0}^{1}{{w_{T_{SYM}}\left( {t - {nT}_{SYM}} \right)} \cdot {\sum\limits_{k = {- 26}}^{26}\left\lbrack {\left( {{\beta_{n} \cdot D_{k,n}} + {p_{n + 1}P_{k}}} \right) \cdot {\exp\left( {j\; 2\;\pi\; k\;{\Delta_{F}\left( {t - {nT}_{SYM} - T_{GI} - N_{CS}^{i_{TX}}} \right)}} \right)}} \right\rbrack}}}}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

The β_(n) is phase rotation value such as +1 or +j, where VHTSIG-Amodulated symbols are phase rotated to ensure VHT preamble detection.For Wider bandwidths such as 40, 80, or 160 MHz, the time domainwaveform of 20 MHz is duplicated in each 20 MHz band frequency of thetransmitted signal.

Further, a time domain waveform for the VHTSIG-B can be represented bythe following equation.

$\begin{matrix}{{r_{{VHTSIG} - B}^{i_{rx}}(t)} = {\frac{1}{\sqrt{N_{STS} \cdot N_{{VHTSIG} - B}^{Tone}}}{\sum\limits_{n = 0}^{1}{{w_{T_{SYM}}\left( {t - {nT}_{SYM}} \right)} \cdot {\sum\limits_{k = {- N_{SR}}}^{N_{SR}}{\sum\limits_{i_{STS} = 1}^{N_{STS}}{{\left\lbrack Q_{k} \right\rbrack_{i_{TX},i_{STS}}\left\lbrack P_{VHTLTF} \right\rbrack}_{i_{STS},1}\left( {D_{k,n} + {p_{n + 1}P_{k}}} \right){\exp\left( {j\; 2\;\pi\; k\;{\Delta_{F}\left( {t - {nT}_{SYM} - T_{GI} - N_{CS}^{i_{TX}}} \right)}} \right)}}}}}}}} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

N_(TX): the number of transmission chains

N_(STS): the number of space time streams

N^(iTx) _(CS): the cyclic shift of an iTX transmission chain

N^(iSTS) _(CS): the cyclic shift of an iSTS space time stream

N^(tone) _(VHTSIG-A) the number of subcarriers used in VHTSIG-A

N^(tone) _(VHTSIG-A): the number of subcarriers used in VHTSIG-B

N_(SR): the number of subcarriers in half of the transmitted signalbandwidth used for VHTSIG-B

β_(n): phase rotation value

T_(SYM): symbol duration

T_(GI): guard interval duration

P_(VHTLTF): VHT-LTF mapping matrix

D_(k,n), p_(n), P_(k), Q_(k): parameters given in Paragraph 20.3 of EEE802.11n/D11.0

Although not shown in FIG. 4, an HT-SIG can be further included in thePLCP frame. The HT-SIG can be disposed after the L-SIG or the VHTSIG-A.If the HT-SIG is further included, an HT-STF and an HT-LTF can befurther included. If a PLCP frame does not provide backwardcompatibility to an L-STA, the L-STF, the L-LTF, and the L-SIG may notbe included. Various PLCP frame formats relating to the above accordingto embodiments of the present invention are described in detail with

reference to block diagrams.

In the above embodiment, although the 20 MHz bandwidth has beendescribed, this is only illustrative. The technical spirit of thepresent invention can be applied to a bandwidth of 40 MHz or more.Further, the technical spirit of the present invention can be applied toa structure in which a plurality of the 20 MHz bandwidths or the 40 MHzbandwidths is combined together.

In the 40 MHz bandwidth, an L-STF, an L-LTF, an L-SIG, and VHTSIG-A inwhich beam-forming is not used uses 104 subcarriers every OFDM symbol inorder to support an L-STA. VHT-LTF and VHTSIG-B in which beam-forming isused uses 112 subcarriers every OFDM symbol.

In the 40 MHz bandwidth, the L-LTF can be represented by the sequence Tof the following frequency domain in one OFDM symbol.

T_(−58,58)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}

In the 40 MHz bandwidth, the VHT-LTF can be represented by the sequenceVHTT of the following frequency domain in one OFDM symbol.

VHTT_(−58,58)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,−1,−1,−1,1,0,0,0,−1,1,1,−1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}

FIG. 7 is a flowchart illustrating a method of transmitting controlinformation according to an embodiment of the present invention.

An STA or an AP transmits public control information at step S710. Thepublic control information is transmitted omni-directionally withoutusing beam-forming. The public control information is decoded using achannel which is estimated through an L-LTF transmitted in a previousOFDM symbol.

After transmitting the public control information, the STA or the APtransmits STA-specific control information at step S720. TheSTA-specific control information is transmitted to a specific STA (or aspecific user) using beam-forming. A specific STA first receives thepublic control information, acquires information for receivingSTA-specific control information, and then receives the STA-specificcontrol information which has been beam-formed and transmitted. TheSTA-specific control information is decoded using a channel which isestimated through VHT-LTFs transmitted in an OFDM symbol between thepublic control information and the STA-specific control information.

Since all Non-AP STAs and APs within a BSS must be able to receive thepublic control information, the time and frequency domains supported byall the Non-AP STAs and APs within the BSS are allocated to the publiccontrol information. Meanwhile, since the STA-specific controlinformation is received by only a specific STA or a specific AP, thetime/frequency domains supported by the specific STA or the specific APare allocated to the STA-specific control information. Accordingly,backward compatibility can be guaranteed, and higher frequencyefficiency can also be provided to a VHT-STA supporting MU-MIMO.

In the frequency domain, the magnitude of frequency resources of thepublic control information can be smaller than the magnitude offrequency resources of the STA-specific control information. Forexample, the number of subcarriers allocated to the public controlinformation may be smaller than the number of subcarriers allocated tothe STA-specific control information.

In the time domain, the magnitude of time resources of the publiccontrol information may be greater than the magnitude of time resourcesof the STA-specific control information. For example, the number of OFDMsymbols allocated to the public control information may be greater thanthe number of OFDM symbols allocated to the STA-specific controlinformation.

Different amounts of cyclic shifts may be applied to the public controlinformation and the STA-specific control information.

The method of allocating frequency resources, the modulation method, thetransmission method, and the method of applying a cyclic shift inrelation to the control information can be applied to various PLCP frameformats likewise, proposed by the present invention.

FIG. 8 shows an example of a PLCP frame structure. FIG. 8 shows a methodof adding a middle VTF-LTF to an intermediate part in which data aretransmitted in the PLCP frame of FIG. 4.

Although a common WLAN system assumes an indoor environment, apossibility that the common WLAN system will be used in an outdoorenvironment cannot be excluded. For example, the WLAN can be used incampuses, outdoor parking places, etc. The outdoor environment has agreater change in the channel than the indoor environment.

If the amount of data is much and so the transmission interval of thedata is long even though only the Doppler effects are taken intoconsideration, performance is expected to be deteriorated because thereis a possibility that the channel can be changed during the longtransmission interval.

Although the data can be divided and transmitted, there may be overheadfor STFs and LTFs according to the format of a PCLP frame. Accordingly,the performance of channel estimation can be prevented from beingdeteriorated even in a change of a channel environment by adding middleVHT-LTFs for the channel estimation in the middle portion of data.

Whether the middle VHT-LTFs will be transmitted can be informed throughVHTSIG-A or VHTSIG-B.

FIG. 9 shows another example of the PLCP frame structure. FIG. 9proposes a method of adding the last VTF-LTFs to the last of the PLCPframe of FIG. 4.

FIG. 10 is a block diagram showing an example of a VHT-GF PLCP frameformat proposed by the present invention.

The VHT-GF PLCP frame proposed by the present invention is transmittedin order of a VHT-GF-STF (VHT Green Field Short Training Field), aVHT-LTF1 (VHT Long Training Field 1), and VHTSIG-A 1050. The VHT-GF-STFand the VHT-LTF1 include a control signal for frame timing acquisition,AGC (automatic gain control) convergence, and channel estimation. TheVHT-GF-STF, the VHT-LTF1, and the VHTSIG-A 1050 are transmittedomni-directionally. It can be recognized that a channel is being used byreceiving the VHT-GF-STF, the VHT-LTF1, and the VHTSIG-A 1050 in whichthe VHT STAs of a BSS (Basic Service Set) are transmittedomni-directionally.

VHTSIG-B 1060, VHT-LTFs, and a data field which are transmitted afterthe VHTSIG-A 1050 are subjected to SDMA precoding and beam-forming andthen transmitted. The VHTSIG-B 1060, the VHT-LTFs, and the data fieldcan be used to transmit information, individualized every target STA,because they are transmitted to a plurality of the target STAs throughMU-MIMO.

FIG. 10 illustrates that a different cyclic shift can be used as cyclicshift CSD1 up to the VHTSIG-A 1050 and as cyclic shift CSD2 after theVHTSIG-A 1050.

FIG. 11 is a block diagram showing another example of the VHT-GF-PLCPframe format proposed by the present invention.

The VHT-GF-PLCP frame format of FIG. 11 shows an example in which aVHT-LTF2 1154 is further transmitted between the VHTSIG-A and theVHTSIG-B in the VHT-GF-PLCP frame of FIG. 10. The VHT-LTF2 1154 istransmitted anterior to the VHTSIG-B and it thus provides controlinformation to be used for channel estimation which enables a receptionSTA to receive the VHTSIG-B to be subsequently transmitted.

FIG. 12 is a block diagram showing yet another example of theVHT-GF-PLCP frame format proposed by the present invention.

The VHT-GF-PLCP frame format of FIG. 12 shows an example in which aVHT-STF 1252 is further transmitted between the VHTSIG-A and theVHT-LTF2 in the VHT-GF-PLCP frame of FIG. 11. The reason why the VHT-STF1252 is further transmitted is to transmit a control signal so that areception STA can properly compensate for transmission power in AGCwhich can vary according to a change of a transmission method because ofomni-directional beam-forming transmission.

FIG. 13 is a block diagram showing still yet another example of theVHT-GF-PLCP frame format proposed by the present invention.

A VHT-GF-STF, a VHT-LTF1, and one VHTSIG 1350 are transmittedomni-directionally so that they can be received by all the STAs of aBSS. Next, precoding is performed, and a VHT-STF, VHT-LTFs, and a datafield are then subjected to beam-forming and transmitted.

FIG. 14 is a block diagram showing still yet another example of theVHT-GF-PLCP frame format proposed by the present invention.

The VHT-GF-PLCP frame of FIG. 14, like the VHT-GF-PLCP frame of FIG. 13,is used to transmit a VHT-GF-STF, a VHT-LTF1, VHTSIG-A 1450, andVHTSIG-B 1460 omni-directionally so that the VHT-GF-STF, the VHT-LTF1,the VHTSIG-A 1450, and the VHTSIG-B 1460 can be received by all the STAsof a BSS. Next, precoding is performed, and a VHT-STF, VHT-LTFs, and adata field are then subjected to beam-forming and transmitted. In thiscase, when spatially multiplexed data are transmitted to a plurality oftarget STAs, the number of streams through MU-MIMO may be variable.Accordingly, control information may also be variable. In theVHT-GF-PLCP frame format of FIG. 14, the VHT-SIG 1350 of FIG. 13 isdivided into the VHTSIG-A 1450 and the VHTSIG-B 1460, and the VHTSIG-A1450 can indicate information about the magnitude of the VHTSIG-B 1460including control information about each variable target STA.

FIG. 15 is a block diagram showing still yet another example of theVHT-GF-PLCP frame format proposed by the present invention.

The VHT-GF-PLCP frame format of FIG. 15 shows an example in which aVHT-GF-STF, a VHT-LTF1, VHTSIG-A, and VHTSIG-B are transmittedomni-directionally, and then a VHT-STF, VHT-LTFs, and a data field arethen subjected to precoded, beam-forming, and then transmitted. Unlikethe VHT-GF-PLCP frames of FIGS. 10 to 14 in which MU-MIMO transmissionis performed, the VHT-GF-PLCP frame of FIG. 15 shows an example of aPLCP frame in which SU-MIMO transmission is performed. The reason whyboth VHT-SIGs (i.e., the VHTSIG-A and the VHTSIG-B) are transmittedomni-directionally is that there is no problem of collision andinterference between PLCP frames toward different STAs because SDMAtransmission is not performed unlike MU-MIMO transmission.

The VHTSIG-A 1550 and the VHTSIG-B 1560 can include an indicator,indicating whether the VHT-GF-PLCP frame is the VHT-GF-PLCP frame ofMU-MIMO or the VHT-GF-PLCP frame of SU-MIMO, in the form of a subfield.For example, in case where a type subfield, including informationindicative of the type of a VHT-GF-PLCP frame, is set to 0 andtransmitted, a reception STA can recognize the indicator as a SU-MIMOVHT-GF-PLCP frame. Further, in case where the type subfield is set to 1and transmitted, the reception STA can recognize the indicator as anMU-MIMO VHT-GF-PLCP frame.

A VHT-STF including a control signal for compensation in AGC accordingto a change of a transmission method is transmitted after the VHTSIG-B1560. Fields posterior to the VHT-STF are subjected to precoding andbeam-forming and then transmitted.

FIG. 16 is a block diagram showing still yet another example of theVHT-GF-PLCP frame format proposed by the present invention.

The VHT-GF-PLCP frame format of FIG. 16 may be used in case where evenVHTSIG-A needs not to be transmitted to other STAs of a BSS using aVHT-GF-PLCP frame format which can be used in SU-MIMO. Unlike in theexample of FIG. 15, in the VHT-GF-PLCP frame of FIG. 16, all the fieldsare subjected to SDMA precoding and transmitted. Since there is nochange of a transmission method in the transmission of the frame, theVHT-STF may be omitted unlike the PLCP frame format of FIG. 15.

FIG. 17 is a block diagram showing an example of a VHT-mixed PLCP frameformat proposed by the present invention.

The VHT-mixed PLCP frame proposed by the present invention includes atraining field and a signal (SIG) field for legacy STAs. The trainingfield and the signal field for legacy STAs are transmitted anterior to atraining field and a signal field for VHT STAs so that the legacy STAscan know that a channel is being used by receiving the training fieldand the signal field for legacy STAs.

Referring to FIG. 17, an L-STF (legacy short training field) ad an L-LTF(legacy long training field) (i.e., training fields for legacy STAs) arefirst transmitted. The L-STF is used for frame timing acquisition andAGC (automatic gain control) convergence, and the L-LTF is used for asignal field (SIG field) and channel estimation for data demodulation.

The signal field is transmitted posterior to the training fields. Here,an L-SIG for a Non-HT STA and an HT-SIG for an HT STA may betransmitted. The HT-SIG, as in the example of FIG. 10, may betransmitted in the form of one field posterior to the L-SIG or may beincluded in the L-SIG and then transmitted as needed. The L-SIG and theHT-SIG includes Modulation and Coding Scheme (MCS) information necessaryto demodulate and decode the data field subsequently transmitted.

The training fields and the signal field for legacy STAs are firsttransmitted and the fields for VHT STAs are then transmitted. The fieldsfor VHT STAs may include a VHT-STF, a VHT-LTF1, VHT-SIG, VHT-LTFs forchannel estimation with individual STAs, and extension VHT-LTFs. Afterthe training fields and the signal field for VHT STAs are transmitted, adata field is transmitted.

In the example of FIG. 17, the training fields and the signal field forlegacy STAs are subjected to only CSD (cyclic shift delay) withoutprecoding so that they can be recognized by the legacy STAs and thentransmitted omni-directionally. The CSD may be performed before or afterInverse Discrete Fourier Transform (IDFT) in a signal transmissionprocess in order to prevent unwanted beam-forming from being generated.The CSD may be performed every transmitter chain or every spatial streamand may be applied as part of a spatial mapper. Next, the trainingfields, the signal field, and the data field for VHT STAs may besubjected to CSD, precoding, and beam-forming and then transmitted.

FIGS. 18 and 19 are block diagrams showing another example of theVHT-mixed PLCP frame format proposed by the present invention.

The VHT-mixed PLCP frame of FIGS. 18 and 19 have the same field andtransmission sequence as the PLCP frame of FIG. 8. However, theVHT-mixed PLCP frame of FIG. 18 differs from the VHT-mixed PLCP frame ofFIG. 8 in that fields up to VHTSIG-A are omni-directionally and fieldsstarting from VHTSIG-B are subjected to SDMA precoding and transmitted.The VHT-mixed PLCP frame of FIG. 19 differs from the VHT-mixed PLCPframe of FIG. 18 in that fields from a VHT-STF to VHTSIG-A are precodedand transmitted.

FIG. 20 is a block diagram showing yet another example of the VHT-mixedPLCP frame format proposed by the present invention.

Referring to FIG. 20, training fields and a signal field for legacy STAsand a signal field VHT-SIG for VHT STAs are transmittedomni-directionally. Next, a field VHT-STF to a data field are subjectedto SDMA precoding and transmitted. Here, the VHTSIG field includescontrol information for demodulating and decoding data received byreception STAs.

FIG. 21 is a block diagram showing an example of a VHT-mixed-GF PLCPframe format proposed by the present invention.

The VHT-mixed-GF PLCP format of FIG. 21 is effective in the case of aWLAN system composed of only IEEE 802.11n HT STAs and VHT STAs or incase where Non-HT STAs need not to be taken into consideration. In theVHT-mixed-GF PLCP frame, an L-STF, an L-LTF, and an L-SIG are nottransmitted because Non-HT STAs need not to be taken into consideration.However, an HT-GF-STF, an HT-LTF1, and an HT-SIG are first transmittedso that HT STAs can recognize the PLCP frame. Next, a VHTSIG andVHT-LTFs for VHT STAs and a data field are transmitted.

In the VHT-mixed-GF PLCP frame of FIG. 21, the VHT-LTFs and the datafield are transmitted immediately after the HT-SIG and the VHT-SIGwithout a VHT-STF because all fields are precoded and a precoded valueis applied to all the fields.

FIG. 22 is a block diagram showing another example of a VHT-mixed-GFPLCP frame format proposed by the present invention.

Unlike in the example of FIG. 21, fields up to a VHT-SIG are transmittedomni-directionally so that all HT STAs and VHT STAs within a BSS canreceive an HT-SIG and the VHT-SIG. Fields subsequent to the VHT-SIG areprecoded and transmitted. That is, a VHT-STF is first transmitted, andVHT-LTFs and a data frame are then transmitted.

FIG. 23 is a block diagram showing yet another example of a VHT-mixed-GFPLCP frame format proposed by the present invention.

In the VHT-mixed-GF PLCP frame of FIG. 23, an HT-GF-STF, an HT-LTF1, anHT-SIG, and VHTSIG-A are transmitted omni-directionally, and allsubsequent fields are precoded and sequentially transmitted in order ofa VHT-STF, a VHT-LTF1, VHTSIG-B, VHT-LTFs, and a data field. Here, theVHTSIG-A may not be additionally transmitted, and parameters for datademodulation and decoding may be transmitted in the VHTSIG-B. In thiscase, information of a subfield transmitted in the HT-SIG may be reused.A reception STA can demodulate and decode the data field on the basis ofthe information of the VHTSIG-B. Further, some of the fields of the PLCPframe format of FIG. 23 may be omitted as needed, and FIGS. 24 and 25show an example thereof.

FIGS. 24 and 25 show formats in each of which the VHT-STF or theVHT-LTF1 is omitted in the example of FIG. 23 and show examples of theVHT-mixed-GF PLCP frame format which may be modified according to theimplementation of an STA.

FIG. 26 shows an example of a VHT-Mixed-PLCP frame format and thetransmission of the VHT-Mixed-PLCP frame according to an embodiment ofthe present invention.

The VHT-Mixed-PLCP frame 2610 includes TFs (training fields) and SIGs(recognizable by legacy STAs) for the legacy STAs, TFs and SIGs for VHTSTAs, and a data field. As an example of the TFs and the SIGs for legacySTAs, the VHT-Mixed-PLCP frame 2610 of FIG. 26 includes an L-STF (Non-HTshort Training Field) 2612, an L-LTF (Non-HT Long Training Field) 2614,an L-SIG (Non-HT SIGNAL Field) 2616, and an HT-SIG (HT SIGNAL field)2618.

The L-STF 2612 is used for frame timing acquisition and AGC (automaticgain control) convergence. The L-LTF 2614 is used for channel estimationfor demodulating the L-SIG 2616 and data. The L-SIG 2616 includesinformation for demodulating and decoding subsequent data. The HT-SIG2618 is a SIG field for an HT STA and may be included in the L-SIG 2616and transmitted. The L-STF 2612, the L-LTF 2614, and the L-SIG 2616 aretransmitted anterior to other fields so that legacy STAs can recognizethem and can know that a channel is being used.

The VHT-Mixed-PLCP frame 2610 according to the embodiment of the presentinvention includes a VHT-STF 2622 for VHT STAs, a VHT-LTF1 2624, twoVHT-SIGs (i.e., VHTSIG-A 2630 and VHTSIG-B 2640), and VHT-LTFs 2650-1, .. . , 2650-L. The VHTSIG-A 2630 includes common information about fieldssubsequently transmitted and the PLCP frame. The VHTSIG-B 2640 mayinclude information individualized every target STA to which data willbe transmitted.

In a method of transmitting frames according to an embodiment of thepresent invention, the VHT-Mixed-PLCP frame 2610 is first transmitted,and N number of GF-PLCP frames 2690-1 to 2690-N are then transmitted.The VHT-Mixed-PLCP frame 2610 includes transmission time informationabout the VHT-Mixed-PLCP frame 2610 and N number of the VHT GF-PLCPframes 2690-1 to 2690-N. Legacy STAs and VHT STAs which are nottransmission target STAs can know that a channel is being used throughthe VHT-Mixed-PLCP frame 2610 and sets an NAV and defers channel accessduring the time for which the channel is used on the basis of thetransmission time information included in the VHT-Mixed-PLCP frame 2610.To this end, fields before the VHTSIG-A 2630 in the VHT-Mixed-PLCP frame2610 are transmitted without SDMA precoding so that they can berecognized by all STAs including legacy STAs, and only fields subsequentto the VHTSIG-A 2630 are subjected to SDMA precoding and transmitted.

Legacy STAs and VHT STAs which are not transmission target STAs may notrecognize N number of the GF-PLCP frames 2690-1 to 2690-N transmittedafter the VHT-Mixed-PLCP frame 2610, but may set an NAV and deferchannel access during the time for which all the VHT-Mixed-PLCP frame2610 and N number of the GF-PLCP frames 2690-1 to 2690-N are transmittedon the basis of the transmission duration information included in theVHT-Mixed-PLCP frame 2610. Accordingly, malfunction can be prevented.

FIG. 27 is a block diagram showing a VHT-GF-PLCP frame format accordingto an embodiment of the present invention.

The VHT-GF-PLCP frame 2700 includes a VHT-GF-STF 2710, a VHT-LTF1 2720,two VHT-SIG fields (i.e., VHTSIG-A 2730 and VHTSIG-B 2740), N number ofVHT-LTFs 2750-1, . . . , 2750-N, and a data field DATA. In the exampleof FIG. 27, the VHTSIG-A 2730 and the VHTSIG-B 2740 are consecutivelytransmitted, but only illustrative. The VHTSIG-B 2740 may be transmittedimmediately after the VHTSIG-A 2730 or may be transmitted after theVHTSIG-A 2730. In the VHT-GF-PLCP frame 2700 according to the presentinvention, the VHT-GF-STF 2710, the VHT-LTF1 2720, and the VHTSIG-A 2730are transmitted omni-directionally so that all VHT STAs can listen tothem. The VHTSIG-B 2740 and data subsequently transmitted can besubjected to SDMA precoding and beam-forming and then transmitted. TheVHTSIG-A 2730 includes common information about subsequent SDMAtransmission. For example, the VHTSIG-A 2730 may include commoninformation about SDMA transmission duration so that third STAs (i.e.,not transmission target STAs) may set an NAV during the SDMAtransmission duration. The VHTSIG-B 2740 has a parameter value settherein or includes the parameter value which is used for SDMAtransmission to each transmission target STA. For example, an MCS index,a channel bandwidth, the number of spatial streams, and so on may be setand included in the VHTSIG-B 2740 on an STA basis and then transmitted.

The VHTSIG-B 2740 and the data subsequently transmitted are subjected toSDMA precoding and beam-forming and then transmitted. Accordingly, athird STA (i.e., not a transmission target STA) does not receive theVHTSIG-B 2740 field and the data subsequently transmitted, but canrecognize a corresponding preamble by receiving the fields up to theVHTSIG1 2730.

In Single User (SU)-MIMO, a GF-PLCP frame may use one VHTSIG. This isbecause since SDMA transmission is not performed in the SU-MIMO,problems, such as collision and interference between PLCP frames headingfor different STAs, are not generated. In order to identify GF-PLCPframes in the SU-MIMO and the MU-MIMO, a type subfield indicative of atransmission type may be included in the VHTSIG-A 2730 and the VHTSIG-B2740. In case where a configuration value of the type subfield indicatestransmission using the SU-MIMO method, only one VHTSIG field is used. Incase where a configuration value of the type subfield indicatestransmission using the MU-MIMO method, two VHTSIGs (i.e., VHTSIG1 andVHTSIG2) are used. As described above, the VHTSIG1 of the two VHTSIGs isused to detect and recognize the preamble of a PLCP frame which isomni-directionally transmitted and being transmitted by STAs within aBSS. Further, the VHTSIG2 of the two VHTSIGs has information about MCSindex values for spatial streams heading for respective transmissiontarget STAs, a channel bandwidth, the number of spatial streams, etc.

FIG. 28 shows an example of a PLCP frame format according to anembodiment of the present invention.

The PLCP frame of FIG. 28 has a VHT-mixed frame format and includesfields L-STF, L-LTF, L-SIG, and HT-SIG for legacy STAs. The fieldsL-STF, L-LTF, L-SIG, and HT-SIG have the same function as describedabove.

FIG. 28 shows the example in which an AP transmits 5 spatial streams totwo STAs STA1 and STA2 using the MU-MIMO method, the first STA STA1receives 3 spatial streams, and the second STA STA2 receives 2 spatialstreams. Here, the number of STAs (i.e., targets of MU-MIMOtransmission) and the number of spatial streams transmitted to the STAsare only illustrative, and the present invention is not limited thereto.

The PLCP frame of FIG. 28 includes a plurality of VHTSIG fields (e.g.,VHTSIG1 and VHTSIG2), each including control information about an STA(i.e., a target of MU-MIMO transmission). That is, the number of VHTSIGfields can be equal to or greater than the number of STAs (i.e., targetsof MU-MIMO transmission).

In the example of FIG. 28, the field VHTSIG1 includes controlinformation about the STA1 (i.e., a target of MU-MIMO transmission), andthe field VHTSIG2 includes control information about the STA2 (i.e., atarget of MU-MIMO transmission).

The VHTSIG field allocated to each STA may consist of several VHTSIGfields, such as VHTSIG1 to the VHTSIGN. For example, the HT-SIG field ofthe IEEE802.11n standards may include two HT-SIGs, which are transmittedin two OFDM symbols. The number of OFDM symbols of the VHTSIG field tobe transmitted can be represented by the number of STAs spatiallymultiplexed using MU-MIMO and a function of the number of spatiallymultiplexed streams.

In the example of FIG. 28, the two VHTSIG fields are illustrated, butduration in which the VHTSIG fields are transmitted is increased with anincrease in the number of STAs (i.e., targets of MU-MIMO transmission).If an AP transmitting eight streams is operated together with eight 1Rx-STAs through MU-MIMO, eight VHTSIG fields (i.e., VHTSIG1 to VHTSIG8)have to consecutively transmitted.

In this case, layer index indication, informing which stream will bereceived by an STA (i.e., a target of MU-MIMO transmission), isrequired. To this end, the VHTSIG field may include an indication bit,indicating control information for a specific one of STAs (i.e., aplurality of targets of MU-MIMO transmission).

An LTF is subjected to code multiplexing and transmitted through aplurality of spatial streams at the same time. The number of LTFstransmitted may be changed in order to provide LTF orthogonality andthus represented by LTFx in the example of FIG. 28.

A method of adding a unique identification signal for controlinformation about each of the STAs to the VHTSIG field or a method ofperforming bit pattern masking (i.e., exclusive OR to parity bits) on anidentification value, identifying an STA, for the CRC parity bit of theVHTSIG field may be used as a method of indicating where controlinformation for each of STAs (i.e., a plurality of targets of MU-MIMOtransmission) is contained in which VHTSIG field may include. In thiscase, the identification value or signal may be an MAC address or anassociation ID of a target STA.

A cyclic shift used up to the VHTSIG field can differ from cyclic shiftssubsequently used. Fields transmitted after the VHTSIG field can besubjected to precoding and beam-forming and then transmitted.

FIG. 29 shows another example of the PLCP frame format according to anembodiment of the present invention. The PLCP frame format of FIG. 29has a VHT GF PLCP format and it is basically the same as that of FIG.28. However, since legacy STAs need not to be taken into consideration,fields (e.g., L-STF, L-LTF, L-SIG, and HT-SIG) for the legacy STAs maybe omitted, and all fields may be subjected to beam-forming andtransmitted.

FIGS. 30 and 31 show yet another example of the PLCP frame formataccording to an embodiment of the present invention.

FIG. 30 shows a VHT-mixed PLCP frame format, and FIG. 31 shows a VHT GFPLCP frame format.

In the PLCP frame of FIG. 30, fields (i.e., L-STF, L-LTF, L-SIG, andHT-SIG) for legacy STAs are transmitted omni-directionally. Next,subsequent fields starting from VHTSIG fields, each including controlinformation about the STA, can be subjected to beam-forming every STAand then transmitted. Accordingly, in the VHT-mixed PLCP frame format ofFIG. 30, the VHTSIG fields are transmitted after fields VHT-STF in eachof which an AGC gain has been taken into consideration are transmitted.That is, the field HT-SIG and the fields VHTSIG are not consecutivelytransmitted.

In case where the VHTSIG fields of FIGS. 28 and 29 support MU-MIMOunlike an overlapped format, the transmission duration of the VHTSIGfield is not changed according to the number of STAs. Further, if an APproperly performs beam-forming (e.g., using a block diagonalizationscheme) for each STA (i.e., a target of MU-MIMO transmission), thecorresponding STA does not interfere with other STAs because it canrecognize only its own stream irrespective of a total number of streams.Accordingly, each STA does not know that it is operated according to theMU-MIMO method and considers that it is operated according to theSU-MIMO method using a small number of spatial streams.

FIGS. 30 and 31 show the examples in which an AP pairs two STAs (i.e.,STA1 and STA2) for MU-MIMO. The STA1 receives 3 streams and receives 4VHT-LTFs for channel measurement. The STA2 receives 2 streams andreceives 2 VHT-LTFs for channel measurement.

In this case, an LTF mapping matrix P can be represented by Equations 4to 6.

Equation 4 shows an example of the LTF mapping matrix which may be usedwhen 2 LTFs are measured, Equation 5 shows an example of the LTF mappingmatrix which may be used when 3 LTFs are measured, and Equation 6 showsan example of the LTF mapping matrix which may be used when 4 LTFs aremeasured.

$\begin{matrix}{P = \begin{bmatrix}1 & {- 1} \\1 & 1\end{bmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack \\{P = \begin{bmatrix}1 & {- 1} & 1 & 1 \\1 & 1 & {- 1} & 1 \\1 & 1 & 1 & {- 1}\end{bmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 5} \right\rbrack \\{P = \begin{bmatrix}1 & {- 1} & 1 & 1 \\1 & 1 & {- 1} & 1 \\1 & 1 & 1 & {- 1} \\{- 1} & 1 & 1 & 1\end{bmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 6} \right\rbrack\end{matrix}$

FIG. 31 shows an example of a VHT GF PLCP frame format. In this frameformat, fields for legacy STAs which are non-overlapped parts areomitted in the example of FIG. 30. Further, since all fields aresubjected to precoding and beam-forming and then transmitted, thetransmission of VHT STF fields for controlling an ACG gain may beomitted.

In the examples of FIGS. 30 and 31, if an AP can transmit 5 or morestreams, beams for STAs can be transmitted without interference witheach other. However, if a total number of RXs of STAs (i.e., targets ofMU-MIMO transmission) is greater than the number of streams that can bereceived, performance loss can be generated because the beams are notproperly formed. For example, it is assumed that when an AP transmits 4streams, an STA1 has 3 Rx antennas and an STA2 has 2 Rx antennas.Assuming that the AP forms a beam in order to transmit 2 streams to eachSTA, the STA1 experiences such performance loss.

Here, the performance loss can be caused by various factors. Wheninterference zero forcing beam-forming used by the AP is performed, atransmission signal will not be transmitted to each STA withoutinterference because of the limit of finite word length precision andchannel coefficient estimation error. Here, the finite word lengthprecision problem refers to a problem occurring because of informationlost when signal information is quantized into digital data in a currentwireless modem. Further, in a state in which spatial interferenceleakage exists, channel estimation can be erroneously performed by LTFsfor different STAs composed of the same sequence. Accordingly,performance loss can be generated because data demodulation is notnormally performed.

Meanwhile, if LTF indications for the streams of different STAs areincluded in its own VHTSIG field although interference is introducedfrom the different STAs (or if indication for its own LTF is included inthe VHTSIG field), interference can be cancelled using a proper receiver(e.g., an MMSE receiver). As described above, there is proposed a methodof including LTF indications for different STAs except its own STA inthe VHTSIG field.

Further, in order to improve the performance of channel estimation in astate in which interference occurs between different STAs because ofsignals for the different STAs, there is proposed a method oftransmitting different sequences to an LTF and other signals (e.g., STFand VHTSIG) through STA-specific scrambling.

As a detailed embodiment, a scrambling code can be generated using theassociation ID of an STA. In this case, STA-specific scrambling needsnot to be necessarily different every STA, and the STAs have only tohave different identification classifications for scrambling signalswhich have been paired through the MU-MIMO method and spatiallymultiplexed at the same time.

This method can be applied to all schemes constructed by overlapping.Accordingly, if the overlapping scheme is included in all PLCP frameformats subsequently proposed, a pertinent indication can be included ina VHTSIG field, as needed, without special mention, and a combination offields LTF, STF, and SIG may be scrambled.

The overlapping scheme is advantageous in that it can maintain properoverhead because the symbol duration of the field VHTSIG does not varyaccording to the number of STAs. A non-overlapping scheme isadvantageous in that it can detect necessary spatial streams using allVHT-LTFs which are transmitted on the assumption that STAs paired forMU-MIMO can know their streams allocated thereto. Accordingly, there isproposed a PLCP frame format in which VHTSIGs are transmitted using theoverlapping scheme and VHT-LTFs are transmitted using thenon-overlapping scheme by making the best use of the advantages.

FIGS. 32 to 35 show examples of a PLCP frame format in which VHTSIGs aretransmitted using the overlapping scheme and VHT-LTFs are transmittedusing the non-overlapping scheme. FIGS. 32 and 33 illustrate cases inwhich when a beam-forming matrix multiplied to the VHTSIG differs from abeamforming matrix multiplied to the VHT-LFT, a VHT-STF for controllingan AGC gain is required before the VHT-LTFs. FIGS. 34 and 35 show casesin which the VHTSIGs and the VHT-LTFs are subjected to beam-formingusing the same beam-forming matrix and transmitted and in which VHT-STFsare not required before the VHT-LTFs are transmitted. FIGS. 32 and 34show examples of a VHT-mixed PLCP frame format, and FIGS. 33 and 35 showexamples of a VHT GF PLCP frame format.

In the examples of FIGS. 32 to 35, it is assumed that one RX antenna isturned on before a reception STA detects the VHTSIG field. The receptionSTA can know information about its own stream and about a total numberof streams by measuring the VHTSIG field using the VHT-LTF and readingthe VHTSIG field. Meanwhile, since one antenna is assumed before theVHTSIG field is read, a diversity gain which can be obtained with anincrease of the number of RXs may not be used.

FIGS. 36 and 37 show PLCP frame formats into which a common VHTSIG fieldhas been introduced.

FIG. 36 shows an example of a VHT-mixed PLCP frame format, and FIG. 37shows an example of a VHT GF PLCP frame format.

The PCLP frame of FIGS. 36 and 37 includes a VHTSIGc field includingcommon control information. The VHTSIGc field is a common VHTSIG field,and it includes common control information about STA1 and STA2. TheVHTSIGc field is transmitted omni-directionally so that all STAs canacquire information about the VHTSIGc field. The VHTSIGc field includesinformation that all STAs is in common informed, such as informationabout streams allocated to each STA and a total number of streams, andthe information is transmitted to each STA through the VHTSIGc. Thefields VHTSIGc and VHT-LTFs are transmitted using the non-overlappingscheme. Next, fields VHTSIG1 and VHTSIG2, each including individualizedcontrol information about each STA, are transmitted using theoverlapping scheme.

In the above several embodiments, when fields are configured using theoverlapping scheme and transmitted to STAs at the same time, a beam mustbe well formed in the direction of each STA for the purpose of a normaloperation and recognized as SU-MIMO from a viewpoint of the STA. Inother words, since LTFs in other STAs do not function as interference,the corresponding STA does not need to take whether other STAs existinto consideration.

However, if interference with other STAs is generated for some reasons,it may not be easy to distinguish its own field allocated thereto fromfields allocated to other STAs. For example, in the case of the PLCPframe formats of FIGS. 32 to 35, three VHT-LTFs are overlapped with theVHT-LTFs by a P matrix and transmitted to the STA1, and two VHT-LTFs areoverlapped with the VHT-LTFs by a p matrix and transmitted to the STA2.In this case, in an 802.11n system supporting only SU-MIMO, an LTF OFDMsymbol is given a fixed pattern consisting of {−1, 1}. Accordingly, theVHT-LTF of the STA1 and the VHT-LTF of the STA2 have OFDM symbols of thesame pattern. If an ideal beam is formed, three VHT-LTFs have to berecognized in the STA1 and two VHT-LTFs have to be recognized in theSTA2. However, the VHT-LTFs of the STA2 can be detected in the STA1, forsome reasons. For example, all the five VHT-LTFs can be recognized inthe STA1. In such a case, from a viewpoint of the STA1, there is nomethod of sorting out VHT-LTFs received through interference if aspecial indication method is not supported. In order to solve theproblem, there is proposed a method of an STA to distinguish its ownVHT-LTFs from other VHT-LTFs.

In accordance with an embodiment of the present invention, a scramblingcode can be applied to fields, such as LTFs and VHTSIG transmitted toSTAs in order to support MU-MIMO. In this case, sequences used in theSTAs can be orthogonal to each other, or they should have at least goodcorrelation characteristic. Further, an STA can distinguish its own LTFsor VHTSIG from LTFs or VHTSIGs for other STAs although it receives theLTFs or VHTSIGs for other STAs serving as interference. Accordingly,there is an interference suppression effect. When the scramblingsequence is initialized, an ID (e.g. an Association ID (AID)) which canbe replaced with an STA ID or an STA ID, STA temporary numbering or thelike can be used. In case where a method using the STA temporarynumbering is used, STAs can be numbered, and scrambling sequences can beinitialized using the numbered values and then applied to overlappingfields.

FIG. 38 is a block diagram showing another example of a radio apparatusin which an embodiment of the present invention is implemented. Theradio apparatus 3800 can be an AP or a non-AP station.

The radio apparatus 3800 includes a processor 3810, memory 3820, and atransceiver 3830. The transceiver 3830 transmits and receives a radiosignal and has the physical layer of IEEE 802.11 implemented therein.The transceiver 3830 supports MIMO transmission through multipleantennas. The processor 3810 is coupled to the transceiver 3830 andconfigured to implement the MAC layer and the physical layer of IEEE802.11. When the processor 3810 processes the operation of atransmission station from among the above methods, the radio apparatus3800 becomes the transmission station. When the processor 3810 processesthe operation of a reception station from among the above methods, theradio apparatus 3800 becomes the reception station.

In the PLCP sublayer of a transmission station implemented in theprocessor 3810, a PLCP preamble is added to a PSDU, transmitted by theMAC layer, on the basis of the above-described PLCP frame format andthen transmitted to the processor 3810 or a PMD sublayer implemented inthe transceiver 3830. In the PMD sublayer, the PLCP frame is transmittedthrough the transceiver 3830 on the basis of a transmission method foreach field of the above-described PLCP frame format using amulti-antenna system. In the PLCP sublayer of a reception stationimplemented in the processor 3810 of the reception station, the PLCPpreamble is removed on the basis of the above-described PLCP frameformat, and the PSDU is transmitted to the MAC layer implemented in theprocessor 3810 of the reception station.

The processor 3810 or the transceiver 3830 or both can include anApplication-Specific Integrated Circuit (ASIC), other chipset, a logiccircuit, and/or a data processor. The memory 3820 can include Read-OnlyMemory (ROM), Random Access Memory (RAM), flash memory, a memory card, astorage medium and/or other storage device. When the above embodimentsare implemented in software, the above schemes can be implemented usinga module (or process or function) for performing the above functions.The module can be stored in the memory 3820 and executed by theprocessor 3810. The memory 3820 can be placed inside or outside theprocessor 3810 and coupled to the processor 3810 using a variety ofwell-known means.

While the invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

The invention claimed is:
 1. A method of transmitting controlinformation by an access point in a Wireless Local Area Network (WLAN)system, the method comprising: generating, by the access point, a firstcontrol sequence including a first very high throughput (VHT) controlsignal containing control information; generating, by the access point,a second control sequence including at least one second VHT controlsignal, wherein when the second sequence is generated for a plurality oftarget stations the second control sequence includes a plurality ofsecond VHT control signals each allowed for a specific target stationamong the plurality of target stations; generating, by the access point,a first time domain signal by modulating the first control sequence;generating, by the access point, a second time domain signal bymodulating the second control sequence; and transmitting, by the accesspoint, the first time domain signal and the second time domain signal tothe plurality of target stations, wherein the first VHT control signalincludes a multi-user multiple input multiple output (MU-MIMO) indicatorindicating whether the second time domain signal is to be transmitted byusing a MU-MIMO scheme or a single-user multiple input multiple output(SU-MIMO) scheme.
 2. The method of claim 1, further comprising: applyinga multi-user multiple input multiple output (MU-MIMO) steering matrix tothe second control sequence to form a pre-coded second control signalbefore modulating the second control sequence when the MU-MIMO indicatorindicates that the second time domain signal is to be transmitted byusing the MU-MIMO scheme.
 3. The method of claim 1, wherein the secondVHT control signal includes a modulation and coding scheme (MCS) indexindicating a MCS used in the step of transmitting.
 4. The method ofclaim 1, wherein the first VHT control signal includes size informationindicating a size of the second VHT control signal.
 5. The method ofclaim 4, wherein the first VHT control signal is transmitted over 48data subcarriers in each of two orthogonal frequency divisionmultiplexing (OFDM) symbols, and the second VHT control signal istransmitted over at least 52 data subcarriers in one OFDM symbol.
 6. Themethod of claim 1, wherein a number of bits for the first VHT controlsignal is 48 and a number of bits for the second VHT control signal isequal to or greater than
 26. 7. A wireless apparatus of transmittingcontrol information in a Wireless Local Area Network (WLAN) system, theapparatus comprising a processor configured to: generate a first controlsequence including a first very high throughput (VHT) control signalcontaining control information; generate a second control sequenceincluding at least one second VHT control signal, wherein when thesecond sequence is generated for a plurality of target stations, thesecond control sequence includes a plurality of second VHT controlsignals each allocated for a specific target station among the pluralityof target stations; generate a first time domain signal by modulatingthe first control sequence; generate a second time domain signal bymodulating the second control sequence; and transmit the first timedomain signal and the second time domain signal to the plurality oftarget stations, wherein the first VHT control signal includes amulti-user multiple input multiple output (MU-MIMO) indicator indicatingwhether the second time domain signal is to be transmitted by using aMU-MIMO scheme or a single-user multiple input multiple output (SU-MIMO)scheme.
 8. The apparatus of claim 7, the processor further configuredto: apply a multi-user multiple input multiple output (MU-MIMO) steeringmatrix to the second control sequence to form a pre-coded second controlsignal before modulating the second control sequence when the MU-MIMOindicator indicates that the second time domain signal is to betransmitted by using the MU-MIMO scheme.
 9. The apparatus of claim 7,wherein the second VHT control signal includes a modulation and codingscheme (MCS) index indicating a MCS used in the step of transmitting.10. The apparatus of claim 7, wherein the first VHT control signalincludes size information indicating a size of the second VHT controlsignal.
 11. The apparatus of claim 10, wherein the first VHT controlsignal is transmitted over 48 data subcarriers in each of two orthogonalfrequency division multiplexing (OFDM) symbols, and the second VHTcontrol signal is transmitted over at least 52 data subcarriers in oneOFDM symbol.
 12. The apparatus of claim 7, wherein a number of bits forthe first VHT control signal is 48 and a number of bits for the secondVHT control signal is equal to or greater than 26.